1. Field of the Invention
The present invention relates to a belt for a continuously variable transmission, which is comprised of metal ring assemblies each having a plurality of endless metal rings laminated one on another, and a plurality of metal elements each having ring slots into which the metal ring assemblies are fitted, the belt being wound around a drive pulley and a driven pulley to transmit a driving force between the pulleys.
2. Description of the Related Art
For explaining how forces generally act on a belt and its components during operation of a continuously variable transmission, reference is now made to FIGS. 7, 8, 10A and 10B which are part of the drawings illustrating an embodiment of the invention.
As shown in FIG. 8, when a metal element 32 supported with a ring slot 35 thereof fitted over a metal ring assembly 31 is inclined forwards in a direction of advancement, a rear end a (in the direction of advancement) of a saddle surface 44 is brought into abutment against an inner surface of the metal ring assembly 31. This causes a problem that a large stress "sgr"H (a hertz-stress) is produced at the rear end a which exerts a deleterious effect on the durability of the metal ring assembly 31. The forward (in the direction of advancement) inclination of the metal element 32 is caused by a tangent frictional force F which the metal element 32 receives on its pulley contact surface, and an urging force E provided between the metal elements 32. This tendency is particularly significant in an exit region of the driven pulley. The reason will be described below.
As can be seen from FIG. 7, a moment generated in the direction of an arrow M by the tangent frictional force F acts to lower the metal element 32 forwards in the direction of advancement about the swinging center C. On the other hand, a radial frictional force xcexcE generated by the urging force E provided between the metal elements 32 generates a moment in the direction of an arrow Mxe2x80x2 on the metal element 32. This moment acts to lower the metal element 32 backwards in the direction of advancement about the swinging center C.
It is known that the tangent frictional force F, which the metal element 32 receives from a drive pulley 6 or a driven pulley 11, is increased in an exit region of the pulley 6 or 11, as shown in FIG. 10A, and such tangent frictional force F reaches four times the value provided when it is assumed that the tangent frictional force F has been distributed averaged over the entire wound region of the pulley 6, 11, for example, for the reason that the pulleys 6, 11 are deformed, whereby the axial thrust is concentrated. In addition, as shown in FIG. 10B, the urging force E between the metal elements 32 assumes a large value in the exit region of the drive pulley 6, but assumes 0 (zero) in the exit region of the driven pulley 11. Therefore, the metal element 32 is liable to be inclined forwards in the direction of advancement to the maximum extent in a location where the tangent frictional force F inclining the metal element 32 forwards in the direction of advancement is the maximum and the urging force E inhibiting the forward (in the direction of advancement) inclination of the metal element 32 assumes 0 (zero), i.e., in the exit region of the driven pulley 11.
When the metal element 32 is greatly inclined forward in the direction of advancement in the exit region of the driven pulley 11 for the above-described reason, the following problem is encountered: A rear end a (in the direction of advancement) of a saddle surface 44 of the ring slot 35 in the metal element 32 is brought into strong abutment against an inner circumferential surface of the metal ring assembly 31 (see FIG. 8), whereby the fatigue life of the metal ring assembly 31 is shortened by the stress "sgr"H generated at the rear end a.
There are conventional known metal elements which are described in Japanese Utility Model Applications Laid-Open Nos. 59-79653 and 63-17353, Japanese Patent Application Laid-Open No. 6-10993 and Japanese Utility Model Application Laid-Open No. 60-107444, and which have a feature in the shape of a saddle surface against which an inner circumferential surface of a metal ring assembly abuts.
In the metal element described in Japanese Utility Model Application Laid-Open No. 59-79653, opposite ends of the saddle surface of the metal element in the direction of advancement are chamfered smoothly., In the metal element described in Japanese Utility Model Application Laid-Open No. 63-17353, a stepped projection is formed at a central (in the direction of advancement) portion of the saddle surface of the metal element, and an arcuate surface having a radius equal to a minimum wound radius of a metal ring assembly is formed at a top surface of the projection. The object of such arrangement is not described in the publication, but it is believed that the inventors have intended to moderate the strong abutment of the opposite (in the direction of advancement) ends of the saddle surface against the inner circumferential surface of the metal ring assembly in a region where a belt for a continuously variable transmission is wound around a pulley. In these metal elements, it is difficult to effectively moderate a hertz stress generated by the abutment of the rear (in the direction of advancement) end of the saddle surface against the inner circumferential surface of the metal ring assembly in an exit region of a driven pulley, because the shape of the saddle surface is symmetrical about its center in the direction of advancement.
In the metal elements described in Japanese Patent Application Laid-Open No. 6-10993 and Japanese Utility Model Application Laid-Open No. 60-107444, the saddle surface of the metal element is formed into an asymmetrical shape such that a rear (in the direction of advancement) portion of the saddle surface is lower in level than a front portion of the saddle surface. The object of such arrangement is to prevent the metal element from falling forwards and backwards by a pitching moment. These metal elements suffer a problem that the shape of the saddle surface is complicated, resulting in an increased processing cost.
Accordingly, it is an object of the present invention to prevent a reduction in the durability of the metal ring assembly due to the inclination of the metal element, without increasing of the processing cost of the metal element.
To achieve the above object, according to a first aspect and feature of the present invention, there is provided a belt for a continuously variable transmission having a drive pulley and a driven pulley, comprising metal ring assemblies each having a plurality of endless metal rings laminated one on another, and a plurality of metal elements each having ring slots into which the metal ring assemblies are fitted. The belt is wound around the drive pulley and the driven pulley to transmit a driving force between both the pulleys, wherein a saddle surface of the ring slot, against which an inner circumferential surface of the metal ring assembly abuts, is formed so that the radius of a rear corner is larger than that of a front corner in a direction of advancement of the metal elements.
With the above arrangement, when the metal element moved away from the driven pulley, falls forwards in the direction of advancement, the comer of the saddle surface of the ring slot located on the rear side in the direction of advancement and having the larger radius is urged against the inner circumferential surface of the metal ring assembly. Therefore, it is possible to maintain the hertz stress generated on the inner circumferential surface of the metal ring assembly to the minimum to enhance the durability of the metal ring assembly.
According to a second aspect and feature of the present invention, each of the metal elements is formed by punching from a metal plate material,-and the rear comer in the direction of advancement is produced by a punching shear drop upon the punching.
With the above arrangement, the corner of the saddle surface of the metal element located on the rear side in the direction of advancement and having the larger radius, is produced as the punching shear drop upon punching of the metal plate material, to form the metal element. Therefore, it is possible to form the corner having the larger radius without carrying-out of a special processing.
According to a third aspect and feature of the present invention, the belt is used in a continuously variable transmission of a vehicle, and the direction of advancement of the metal element is the advancing direction during forward traveling of the vehicle.
With the above arrangement, the corner having the larger radius can be urged against the inner circumferential surface of the metal ring assembly during forward traveling of the vehicle to enhance the durability of the metal ring assembly by ensuring that the advancing direction of the metal element corresponds to the advancing direction during forward traveling of the vehicle, because the frequency of forward traveling of the vehicle is overwhelmingly higher than the frequency of backward traveling of the vehicle.